Host Plant Acceptance by Redlegged Earth Mite, Halotydeus destructor (Tucker) (Acarina: Penthaleidae)

Host plant acceptance behavior of redlegged earth mites (Halotydeus destructor) on cotyledons of subterranean clover (subclover) lines was investigated in the laboratory. Sustained feeding consisted of a series of short bouts of feeding; rather than one long period of feeding. H. destructor preferred to feed at mite-damaged rather than undamaged sites on cotyledons. With time the numbers of mites feeding in aggregations increased, as mites were attracted to damaged patches. Most feeding occurred in aggregations, and mites in such groups benefited by greater weight gain. The sequence of foraging leading to feeding was similar between subclover lines, however, feeding and aggregation activities were markedly reduced on a resistant line. Host plant acceptance occurred during probing but only after some feeding had occurred.     

Antixenotic resistance of subterranean clover cotyledons to redlegged earth mite, Halotydeus destructor

The short-term response of redlegged earth mite, Halotydeus destructor (Tucker) (Acarina: Penthaleidae) to cotyledons of different varieties of subterranean clover (Trifolium subterraneum subsp. subterraneum L.) was assessed by means of paired choice tests, and no-choice tests. H. destructor had lower numbers and fed less on detached cotyledons of resistant than susceptible varieties, yielding a correlation between the numbers of mites and feeding damage to the cotyledons during a three hour test period. For a number of resistant and susceptible varieties, there was a negative correlation between cotyledon deterrence in the three hour choice test and feeding damage to seedling after a two week period. Since the response of the mites to different subclover varieties occurred within three hours, it is concluded that the resistance is based on antixenosis.No evidence was obtained for antifeedant activity in organic solvent extracts from the variety DGI007 (resistant) in comparison with those from the variety Dalkeith (susceptible). Water soluble compounds from DGI007 cotyledons were preferred by mites, in feeding tests in terms of numbers, over those from Dalkeith (susceptible). Squeezed sap from the cotyledons of both varieties showed the same effects on mites as 5% glucose and were more phagostimulatory than water extracts. Mechanically damaged cotyledons of Dalkeith and DGI007 attracted more mites than the undamaged counterparts. The toughness of cotyledons in 17 varieties of T. subterraneum subsp. subterraneum was measured with a manual penetrometer. Results showed a negative correlation between toughness values and mite feeding damage scores (r²=0.752) for all varieties except S3615D (resistant). This implies a likely involvement of epidermal toughness as a contributor in the antixenotic resistance of these varieties. Other mechanisms may be involved in the resistance of S3615D.     

Assays for the effects of volatile compounds from artificially damaged cotyledons of subterranean clover on the redlegged earth mite, Halotydeus destructor

The effects on the redlegged earth mite (Halotydeus destructor) (Acarina: Penthaleidae) of volatile compounds released from artificially damaged cotyledons of subterranean clover (Trifolium subterraneum L.), a host plant, were investigated with different assays. Mites were repelled by the volatile compounds in flask tube and in trimmed cotyledon disc tests. No differences could be shown between different tissue amounts and between resistant and susceptible varieties. When a membrane sachet was used containing either 2-(E)-hexenal, a compound produced by damaged subclover cotyledons, or the total volatile compounds collected from damaged cotyledons of Dalkeith (susceptible) admixed with 1% glucose, mites gathered to low but were deterred by high concentrations of the compounds. Volatile compounds collected from the resistant variety DGI007 were more deterrent than those from the susceptible Dalkeith. Membrane sachets containing 30 p.p.m. of 2-(E)-hexenal and 1% glucose were less preferred than cotyledons of Dalkeith (susceptible) but were not different from DGI007 (resistant). By increasing the glucose concentration to 5%, the membrane sachets with 30 p.p.m. of 2-(E)-hexenal were preferred to cotyledons of either variety. The results indicate that the membrane feeding technique provides a sensitive way of assaying volatile compounds from damaged subclover cotyledons against the mite.     

Herbivore-induced indirect defense across bean cultivars is independent of their degree of direct resistance

We tested the extent to which resistance of common bean (Phaseolus vulgaris) cultivars to the spider mite Tetranychus urticae parallels the extent to which these plants display indirect defenses via the induced attraction of the predatory mite Phytoseiulus persimilis. First, via field and greenhouse trials on 19 commercial bean cultivars, we selected two spider mite-resistant (Naz and Ks41128) and two susceptible (Akthar and G11867) cultivars and measured the spider mite-induced volatiles and the subsequently induced attraction of predatory mites via olfactory choice assays. The two major volatiles, 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) and (Z)-3-hexenyl-acetate, were induced in the resistant but not in the susceptible cultivars. However, uninfested susceptible cultivars emitted these volatiles at levels similar to those of mite-infested resistant cultivars. Significant induction of several minor components was observed for all four cultivars except for the infested-susceptible cultivar G11867. Both, the spider mite-resistant cultivar Naz and the susceptible cultivar G11867, attracted more predatory mites when they were infested. In contrast, spider mites induced increased emission of two major and five minor volatiles in Ks41128, but predatory mites did not discriminate between infested and uninfested plants. Overall, the attraction of predatory mites appeared to correlate positively with the presence of TMTT and (Z)-3-hexenyl acetate and negatively with β-caryophyllene and α-pinene in the bean headspace. Taken together, our data suggest that resistance and attraction of natural enemies via induced volatiles are independent traits. We argue that it should be possible to cross predator-attraction promoting traits into resistant cultivars that lack sufficiently inducible indirect defenses.     

Ultrastructure and anatomy of nematode-induced syncytia in roots of susceptible and resistant sugar beet

Summary Using susceptible and resistant sugar beet lines, comparative analyses of root histology and ultrastructure were made during invasion by nematodes and the induction and formation of specific feeding structures (syncytia).The resistant line carried the resistance geneHs1^pro–1.Nematodes were able to invade and induce functional syncytia in roots of resistant and susceptible lines. However, syncytia in resistant roots were smaller and less hypertrophied. The vacuolar system of syncytia in susceptible plants contained many small vacuoles. In resistant plants vacuoles were larger but less numerous. Smooth endoplasmic reticulum prevailed in syncytial protoplasts of susceptible plants, whereas almost only rough endoplasmic reticulum occurred in syncytia in resistant plants. The most conspicuous and hitherto undescribed trait of syncytia in resistant roots was the initial appearance of loose, and later compact, aggregations of the endomembrane system which composed most of the endoplasmicreticulum system of syncytia at later stages. Syncytia in resistant plants usually degraded before the nematodes reached their adult stage. The appearance of membrane aggregations and the other resistance-specific features are discussed in relation to their possible effects on syncytium function and role in nematode resistance.Abreviations DAI days after inoculation - ER endoplasmic reticulum - ISC initial syncytial cell - J2 second-stage juvenile - MA membrane aggregations - RER rough endoplasmic reticulum - SER smooth endoplasmic reticulum     

Behavioral responses of the whitebacked planthopperSogatella furcifera (Homoptera: Delphacidae) on rice plants whose odors have been masked

In a two-choice test, moreS. furcifera females settled more often on exposed plants than on parafilm-masked ones, regardless of the susceptibility of rice varieties. This indicates that rice volatiles play an important role in the insect's short-range orientation to its host. The fact that more insects settled on exposed resistant Rathu Heenati (RHT) than to masked susceptible Taichung Native 1 (TN1) suggests that there must be certain common volatiles released by both varieties. Few females landed on masked plants of either RHT or TN1. This implies that the insect could not recognize at a distance that a plant was resistant or susceptible without olfactory stimuli.S. furcifera excreted less honeydew on masked plants than on exposed ones for both varieties and more on masked TN1 than on exposed RHT. The electronic monitoring of feeding behavior demonstrates that the insect made more frequent probes and had shorter phloem ingestion durations on exposed RHT than on exposed TN1 and on masked RHT than on masked TN1. Moreover, the insect had longer phloem ingestion durations on masked TN1 than on exposed RHT. These results suggest that volatile chemicals given off by resistant RHT plants have a negative effect on feeding.     

Genotypic variability and relationships between mite infestation levels, mite damage, grooming intensity, and removal of Varroa destructor mites in selected strains of worker honey bees (Apis mellifera L.)

The objective of this study was to demonstrate genotypic variability and analyze the relationships between the infestation levels of the parasitic mite Varroa destructor in honey bee (Apis mellifera) colonies, the rate of damage of fallen mites, and the intensity with which bees of different genotypes groom themselves to remove mites from their bodies. Sets of paired genotypes that are presumably susceptible and resistant to the varroa mite were compared at the colony level for number of mites falling on sticky papers and for proportion of damaged mites. They were also compared at the individual level for intensity of grooming and mite removal success. Bees from the "resistant" colonies had lower mite population rates (up to 15 fold) and higher percentages of damaged mites (up to 9 fold) than bees from the "susceptible" genotypes. At the individual level, bees from the "resistant" genotypes performed significantly more instances of intense grooming (up to 4 fold), and a significantly higher number of mites were dislodged from the bees' bodies by intense grooming than by light grooming (up to 7 fold) in all genotypes. The odds of mite removal were high and significant for all "resistant" genotypes when compared with the "susceptible" genotypes. The results of this study strongly suggest that grooming behavior and the intensity with which bees perform it, is an important component in the resistance of some honey bee genotypes to the growth of varroa mite populations. The implications of these results are discussed.     

Relative resistance of transgenic tomato tissues expressing high levels of tobacco anionic peroxidase to different insect species.

Different parts of genetically transformed tomato (Lycopersicon esculentum L.) plants that express the tobacco anionic peroxidase were compared for insect resistance with corresponding wild type plants. Leaf feeding by first instar Helicoverpa zea and Manduca sexta was often significantly reduced on intact transgenic plants and/or leaf disks compared to wild type plants, but the effect could depend on leaf age. Leaves of transgenic plants were generally as susceptible to feeding damage by third instar Helicoverpa zea (Boddie) and Manduca sexta (L.) as wild type plants. Green fruit was equally susceptible to third instar larvae of H. zea in both type plants, but fruit of transgenic plants were more resistant to first instar larvae as indicated by significantly greater mortality. Basal stem sections were more resistant to neonate larvae of H. zea and adults of Carpophilus lugubris Murray compared to wild type plants as indicated by significantly greater mortality and/or reduced feeding damage. Thus, tobacco anionic peroxidase activity can increase plant resistance to insects in tomato, a plant species closely related to the original source plant species, when expressed at sufficiently high levels. However, the degree of resistance is dependent on the size of insect and plant tissue involved.     

绿盲蝽取食与机械损伤对棉花叶片内防御性酶活性的影响

为探明绿盲蝽Apolygus lucorum (Mayer-Dür)取食和机械损伤对不同抗性棉花叶片内主要防御酶活性的影响以及防御酶与棉花抗绿盲蝽性的关系,以棉花3个不同抗性品系为材料,室内条件下测定绿盲蝽取食和机械损伤处理后棉叶中苯丙氨酸解氨酶(PAL)、过氧化物酶(POD)和多酚氧化酶(PPO)的活性.结果表明:对...     

Phenotypic mechanisms of host resistance against greenbug (Homoptera: Aphididae) revealed by near isogenic lines of wheat

Interactions between biotype E greenbug, Schizaphis graminum (Rondani), and wheat, Triticum aestivum L., were investigated using resistant and susceptible near isogenic lines of the greenbug resistance gene Gb3. In an antixenosis test, the greenbugs preferred susceptible plants to resistant ones when free choice of hosts was allowed. Aphid feeding resulted in quick and severe damage to susceptible plants, which seemed to follow a general pattern spatially and was affected by the position where the greenbugs were initially placed. Symptom of damage in resistant plants resembled senescence. Within-plant distribution of aphids after infestation was clearly different between the two genotypes. Significantly more greenbugs fed on the first (oldest) leaf than on the stem in resistant plants, but this preference was reversed in the susceptible one. After reaching its peak, aphid population on the susceptible plants dropped quickly. All susceptible plants were dead in 10-14 d after infestation due to greenbug feeding. Aphid population dynamics on resistant plants exhibited a multipeak curve. After the first peak, the greenbug population declined slowly. More than 70% of resistant plants were killed 47 d after infestation. Performance of both biotype E and I greenbugs on several Gb3-related wheat germplasm lines were also examined. It seems that the preference-on-stem that was characteristic of biotype E greenbugs on the susceptible plants was aphid biotype- and host genotype-dependent. Results from this study suggested that antixenosis, antibiosis, and tolerance in the resistant plants of wheat might all contribute to resistance against greenbug feeding.     

Variation in the development of stem nematodes, Ditylenchus dipsaci, in susceptible and resistant crop plants

Dicotyledonous plants were reliably inoculated with stem nematodes by placing drops of 1.3% carboxymethylcellulose containing the surface sterilised worms between the cotyledons or in the leaf axils of recently emerged seedlings raised in pots of cool, moist soil. Inoculating onion leaves, bean stems and potato tubers or potato leaves gave variable results and inoculating onion, tulip and narcissus bulbs and lucerne and red clover stems was usually unsuccessful. An attempt to characterise 67 stem nematode populations by the reactions of a number of different plants failed for lack of useful differential hosts. Lucerne was, however, resistant to all but lucerne populations. Multiplication of stem nematode populations varied greatly between cultivars of lucerne or red clover. Some cultivars were resistant to some populations of lucerne or red clover stem nematodes and susceptible to others. These differences could not be ascribed to differences in viability of the nematodes or to differences in success of the inoculations. They indicate the presence of different pathotypes or biotypes in different populations of a so-called ‘host race’ and indicate the need for new resistant cultivars to be tested against a range of populations before they are released for general use. Amongst lucerne cultivars tested, Vertus was resistant to some lucerne stem nematode populations and susceptible to others. The supposedly resistant lucerne cultivars Euver and Lifeuil were as susceptible as was Europe. Amongst red clover cultivars tested, Redhead, Kühn, Changins and Mt Calme were susceptible, Britta, Lucrum and Temara were the least resistant, Renova, Rittinova and Quin were intermediate and susceptible to one or more of the populations tested but Norseman and especially Sabtoron were very resistant.     

Resistance of cabbage (Brassica oleracea capitata group) crops to Mamestra brassicae

Twenty-one cabbage (Brassica oleracea capitata group) varieties, including 16 local varieties and five commercial hybrids, were screened for resistance to the moth Mamestra brassicae L. under natural and artificial conditions in northwestern Spain. Resistance was assessed as the proportion of damaged plants and damaged leaves, leaf feeding injury, and number of larvae present. Correlation coefficients among damage traits showed that a visual scale (general appearance rating) should be a useful indicator of resistance. Most local varieties were highly susceptible to M. brassicae, whereas the commercial hybrids tested were resistant in terms of head foliage consumption and number of larvae per plant. Performance of varieties was similar under natural and artificial infestation although some of them performed differently at each year. Three local varieties (MBG-BRS0057, MBG-BRS0074, and MBG-BRS0452) were highly susceptible at both natural and artificial infestation conditions being MBG-BRS0074 the most damaged variety. Two local varieties (MBG-BRS0402 and MBG-BRS0535) and commercial hybrids were identified as resistant or moderately resistant to M. brassicae. Among them, 'Corazón de Buey' and 'Cabeza negra' were the most resistant and produced compact heads. These varieties could be useful sources of resistance to obtain resistant varieties to M. brassicae or as donors of resistance to other Brassica crops. The possible role of leaf traits, head compactness, and leaf glucosinolate content in relation to M. brassicae resistance is discussed.     

Characteristics of resistance in rice to rice hoja blanca virus (RHBV) and its vector, Tagosodes orizicolus (Muir)

This paper characterises resistance to rice hoja blanca virus (RHBV) which is transmitted by the planthopper Tagosodes orizicolus (Muir). Resistance is expressed as decreased proportion of plants infected compared to susceptible lines, although severity of symptom expression is similar in both types. This resistance is not due to differences in vector feeding behaviour. Vectors reared eight generations on resistant plants showed no increased ability to transmit to resistant lines or decreased ability to transmit to susceptible ones. Longevity of vectors was similar when reared either on virus-resistant or susceptible plants. Incubation period of the virus in resistant plants are significantly longer than in susceptible plants. Resistance increased with plant age in resistant and susceptible cultivars. Increased virus dosage, as determined by increased number of viru-liferous vectors per inoculated plant, caused an increase in transmission to both resistant and susceptible cultivars. However, the ranking of resistant and susceptible remained the same across the experimental range of dosage and plant age. It is concluded that the resistance studied is to virus infection and there is little risk of “breakdown” occurring as a result of genetic or behavioural changes in the vector population. This will permit the use of economic thresholds to planthopper feeding damage with little risk of RHBV epidemic outbreak.     

No fitness cost for wheat's H gene-mediated resistance to Hessian fly (Diptera: Cecidomyiidae)

Resistance (R) genes have a proven record for protecting plants against biotic stress. A problem is parasite adaptation via Avirulence (Avr) mutations, which allows the parasite to colonize the R gene plant. Scientists hope to make R genes more durable by stacking them in a single cultivar. However, stacking assumes that R gene-mediated resistance has no fitness cost for the plant. We tested this assumption for wheat's resistance to Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). Our study included ten plant fitness measures and four wheat genotypes, one susceptible, and three expressing either the H6, H9, or H13 resistance gene. Because R gene-mediated resistance has two components, we measured two types of costs: the cost of the constitutively-expressed H gene, which functions in plant surveillance, and the cost of the downstream induced responses, which were triggered by Hessian fly larvae rather than a chemical elicitor. For the constitutively expressed Hgene, some measures indicated costs, but a greater number of measures indicated benefits of simply expressing the H gene. For the induced resistance, instead of costs, resistant plants showed benefits of being attacked. Resistant plants were more likely to survive attack than susceptible plants, and surviving resistant plants produced higher yield and quality. We discuss why resistance to the Hessian fly has little or no cost and propose that tolerance is important, with compensatory growth occurring after H gene-mediated resistance kills the larva. We end with a caution: Given that plants were given good growing conditions, fitness costs may be found under conditions of greater biotic or abiotic stress.     

Electronically monitored cowpea aphid feeding behavior on resistant and susceptible lupins

The feeding behavior of cowpea aphid, Aphis craccivora Koch (Homoptera: Aphididae) was examined on seedlings of narrow leafed lupin, Lupinus angustifolius L., and yellow lupin, L. luteus L., using electronic monitoring of insect feeding behavior (EMIF). Aphid feeding behavior was first compared between resistant (cv. Kalya) and susceptible (cv. Tallerack) varieties of narrow-leafed lupin. Aphids spent significantly more time in non- penetration and stylet pathway activities, and significantly less time in the sieve element phase on Kalya than on Tallerack, suggesting that feeding deterrence is an important component of aphid resistance in Kalya. Aphid feeding on a susceptible yellow lupin variety (cv. Wodjil) was then compared with that on two resistant lines, one (Teo) with high and the other (94D024-1) with low seed alkaloid content. There were no consistent differences in aphid feeding behavior between Wodjil and Teo. Total, mean and percentage sieve element phase times were significantly lower, and total and percentage times in non-phloem phase were greater on 94D024-1 than on Wodjil, suggesting the possibility of phloem-based deterrence in 94D024-1.     

Increased damage by western flower thrips Frankliniella occidentalis in chrysanthemum intercropped with subterranean clover

Suppressive effects of intercropping on Frankliniella occidentalis (Pergande) infestations have been reported in several crops. However, this study demonstrates that in year-round chrysanthemum, Dendranthema grandiflora Tzvelev, undersowing with subterranean clover, Trifolium subterraneum L., results in an increased thrips feeding damage. In a pot experiment, performed with chrysanthemum plants (cultivars Reagan and Tiger) grown with or without subterranean clover, significantly more leaves with silver and growth damage were found in the chrysanthemum plants with subterranean clover in comparison with the monocropped chrysanthemum plants. Similarly, the degree of deformation of leaf perimeter and leaf surface was higher in the top leaves of the intercropped chrysanthemum plant. In the soil experiment (only performed with Tiger and plants were grown in the soil in the greenhouse) intercropped chrysanthemum plants suffered a higher feeding damage as well. Analysis of the relation between silver or growth damage and the thrips pressure demonstrates that at similar thrips pressure in the intercropped chrysanthemum plants suffered significantly more damage. The higher thrips pressure in the intercropped chrysanthemum only explains the differences in damage partly. Changes in the reaction of chrysanthemum plants to thrips feeding or in the behaviour of the thrips, mediated by the presence of the non-host crop, are discussed. Our explanation is that chrysanthemum plants grown with clover are more susceptible to thrips feeding than monocropped plants. We conclude that undersowing with clover does not contribute to reduce damage by F. occidentalis in year-round chrysanthemum. Also, the influence of crop diversification on a pest cannot be foreseen until the specific characteristics of each individual crop – pest system are studied.     

Feeding behavior and performance of Nasonovia ribisnigri on grafts,detached leaves,and leaf disks of resistant and susceptible lettuce

Aphids are dependent on the phloem sap of plants as their only source of nutrients. Host‐plant resistance in lettuce, Lactuca sativa L. (Asteraceae), mediated by the Nr gene is used to control the lettuce aphid Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). The resistance is located in the phloem; however, the exact mechanism of resistance is unknown. In this study, we investigated whether the resistance factor (or factors) is synthesized in the root or in the shoot. The feeding behavior and performance of avirulent N. ribisnigri were studied on grafts of resistant and susceptible lettuce. In addition, the persistence of resistance in excised lettuce tissue was measured, by studying the feeding behavior and performance of N. ribisnigri on detached leaves and leaf disks of resistant lettuce. It appears that the resistance factor encoded by the Nr gene is produced in the shoots: aphid feeding was reduced on resistant shoots grafted on susceptible roots, whereas aphids were able to feed on grafts of susceptible shoots on resistant roots. Partial loss of resistance was observed after detachment of leaves and excision of leaf disks from resistant plants. Aphids fed longer on excised resistant plant tissue compared with intact resistant plants; however, compared with excised plant tissue of the susceptible cultivar, the time spent on feeding was shorter, indicating resistance was not completely lost. Our findings caution against the use of excised leaf material for aphid resistance bioassays.     

Do plants use airborne cues to recognize herbivores on their neighbours?

Plants show defensive responses after exposure to volatiles from neighbouring plants infested by herbivores. When a plant’s neighbours host only species of herbivores that do not feed on the plant itself, the plant can conserve energy by maintaining a low defence level. An intriguing question is whether plants respond differently to volatiles from plants infested by herbivores that pose greater or lesser degrees of danger. We examined the secretion of extrafloral nectar (EFN) in lima bean plants exposed to volatiles from cabbage plants infested by common cutworm, two-spotted spider mites, or diamondback moth larvae. Although the first two herbivore species feed on lima bean plants, diamondback moth larvae do not. As a control, lima bean plants were exposed to volatiles from uninfested cabbage plants. Only when exposed to volatiles from cabbage plants infested by spider mites did lima bean plants significantly increase their EFN secretion compared with the control. Increased EFN secretion can function as an indirect defence by supplying the natural enemies of herbivores with an alternative food source. Of the three herbivore species, spider mites were the most likely to move from cabbage plants to lima bean plants and presumably posed the greatest threat. Although chemical analyses showed differences among treatments in volatiles produced by herbivore-infested cabbage plants, which compounds or blends triggered the increased secretion of EFN by lima bean plants remains unclear. Thus, our results show that plants may tune their defence levels according to herbivore risk level.     

捕食螨化学生态研究进展

捕食螨是重要的生物防治因子。早在20世纪70年代就发现了捕食螨的性信息素,许多研究证明植物挥发物在捕食螨向猎物定位过程中发挥着至关重要的作用,影响捕食螨寻找猎物的植物挥发物来源于未受害植物、机械损伤植物、猎物危害植物、非猎物危害植物。人工合成的植物挥发物组分对捕食螨具有引诱作用,但引诱活性低于虫害诱导植物释放的挥发性混合物。捕食螨的饲养条件、饥饿程度、学习与经验行为等会影响捕食螨对植物挥发物的反应。介绍了信息素与植物挥发物对捕食螨的作用,并讨论了目前存在的问题和研究前景。     

Environmental conditions affecting the strength of induced resistance against mites in cotton

Summary Spider mites (Tetranychus urticae) were raised on cotton plants that had been damaged by a previous bout of mite feeding (T. turkestani) and on control plants that were previously not exposed to mites. The effect of induced plant responses upon mite populations was variable ranging from a 4-fold reduction in population growth to no difference at all. The strength of induced resistance was greatest when the population growth of mites was low for other, unknown, reasons. When mite population growth on control plants was great, the effects of induced resistance were diminished.Mite population growth was inversely related to levels of initial damage caused by previous feeding. There was no evidence of a damage threshold that needed to be exceeded before induced resistance became effective. Increased levels of initial damage were not associated with morphological changes in the plants.